The present invention relates to a digital video tape recorder and, more particularly, to apparatus for recording a track number along with the digital video and audio signal on a recording track on a recording medium.
Video tape recorders (VTR) for both consumer use and professional use record a time-code in addition to the audio and video signals. The time code typically is a numerical value representing the hour, minute, second and field/frame of recording of the video and audio signals that it accompanies and is employed to detect a desired position on a tape as during a high-speed searching mode. When video and audio signals are recorded, the time code of the preceding signal usually is detected and a new time code which is contiguous with the detected time code is generated and compared to the next-recorded time code, for example, for error detection.
One problem encountered with the above-described video tape recorders is the general inability to ascertain the absolute track number of the magnetic tape when video and audio signals are recorded in different recording modes. Video tape recorders (VHS, BETA, 8 mm etc) are capable of recording a video and audio signal in a standard play (SP) recording mode and a long play (LP) recording mode. An extended play (EP) recording mode is also available in many video tape recorders. When video and audio signals are recorded in different recording modes on the same magnetic tape, two tracks may be identified by the same time code. FIG. 1A illustrates the tracks formed on a magnetic tape recorded in SP mode with their corresponding time codes. Tracks are formed obliquely on a magnetic tape but are shown perpendicularly across the tape to simplify the drawings.
FIG. 1B illustrates the magnetic tape when a portion of the tape shown in FIG. 1A is re-recorded (overwritten) with video and audio signals in LP mode. As shown, tracks 10-13 are recorded in SP mode and subsequent tracks 14-21 are recorded in LP mode. Tracks formed in LP mode are narrower than tracks formed in SP mode, and as a result, LP mode tracks 18-21 have the same time code as following SP mode tracks 18-21 that had been previously recorded. Therefore, several time codes are recorded twice on the same magnetic tape and such time codes cannot serve as absolute indicators of the tape position.
To overcome this problem, there is a known process of determining the remaining recording length of a tape such as disclosed in Japanese Patent Publication No. 1-15955, from which absolute tape position can be determined. According to the disclosed process, the length of the tape is calculated as a function of the measured diameter of the tape wound on a reel (e.g. the take-up reel), the diameter of the hub of the reel, the thickness of the tape, the rotational velocity of the supply reel and the tape travelling speed.
One difficulty encountered in a typical video tape recorder using the above-described process is the general inability to calculate the exact remaining length of the tape because of the existence of winding irregularities, various thicknesses of air layers between the layers of the tape on the reel and the fact that the tape may be wound under different conditions during the course of operation (for example, the tape may be wound during SP playback mode, LP playback mode, CUE/REV mode and fast forward mode). Hence, this process provides merely an approximation of the remaining length of tape.
Another difficulty with video tape recorders using the above-described process is that the tape supply reel must make several rotations before the length of the tape can be calculated. This process adds several seconds to the entire process of inserting a tape into the video tape recorder, lowering the cassette holder, loading the tape, calculating the remaining tape and indicating the remaining time. In addition, when the remaining tape recording time of several tapes is desired, several additional steps are necessary including depressing the eject button, unloading the tape, lifting the cassette holder and removing the tape cassette before a new tape cassette is inserted. This fairly complex procedure requires about 10 seconds (and varies depending upon whether the takeup reel or the supply reel contains more tape) to determine the remaining tape recording time of a single magnetic tape.